This invention relates generally to electrode-type glow discharge devices used for sputter coating or sputter cleaning, and, more particularly, to such devices employing a post cathode of the magnetron type, the cathode being disposed in a vacuum chamber in which there is generated a practically uniform magnetic field having parallel lines of magnetic flux.
The process of cathode sputtering is well known, and has been described in many publications and patents, some of which are listed in U.S. Pat. No. 3,884,793, entitled "Electrode-Type Glow Discharge Apparatus" and issued in the names of Alan S. Penfold and John A. Thornton. In essence, the cathode sputtering process operates to remove atoms from a target electrode with sufficient energy that they can interact with the atomic structure of a work surface, usually referred to as a substrate, and form thereon a permanent coating of the material removed from the target electrode. In a diode sputtering apparatus, the target electrode is connected as a cathode and is placed in a discharge chamber to provide a low-pressure gaseous environment which becomes ionized in the vicinity of the target cathode. Ionized atoms of the gas bombard the target cathode and drive off, that is sputter, atoms of the target material. The substrate to be coated with the target material is positioned in the path of the sputtered atoms, which then recombine on the substrate surface to form a coating having generally the same chemical composition as the target material, although not necessarily the same physical properties.
It is highly desirable in cathode sputtering processes for the ionized gas, i.e., the gas plasma, to be confined to a region close to the cathode. The ions produced in the plasma will then most likely be drawn to the cathode, and not lost to the walls of the surrounding discharge chamber. The aforementioned patent is directed to apparatus for maintaining one or more plasma traps in the vicinity of a cathode. The plasma is trapped by an appropriately shaped magnetic field, which also has the effect of inducing electrons in the plasma to follow a spiral path, thereby encouraging a relatively large number of collisions with neutral gas atoms, and producing ions at a desirable high rate.
In accordance with the teachings of the aforementioned patent, the cathode may take the form of a cylinder or post with outwardly projecting end flanges, and the plasma region is confined to a cylindrical sheet around the exterior of the cathode. Such cathodes are in widespread use, and provide a source of metallic and non-metallic materials for the vacuum deposition of films onto substrates placed around the cathode. In order for a post cathode to operate in the magnetron mode, in which film deposition rates are relatively high, a magnetic field must be provided in the vicinity of the surface of the cathode. Preferably, the magnetic flux lines must be parallel to the longitudinal axis of the cathode, in order to promote uniform sputter erosion of material from the surface. The magnetic field is typically generated by a solenoid, or an array of solenoids, disposed in a co-axial fashion surrounding the post cathode. So long as the solenoid array is made appreciably longer than the cathode, field non-uniformities near the ends of the cathode can be minimized.
An increasingly large number of applications of sputter coating techniques now require very large vacuum chamber systems, sometimes with relatively large length-to-diameter ratios. The use of extremely long solenoid arrays in such large chambers, to achieve a perfectly uniform field, poses significant practical difficulties, and increases the cost of the vacuum chamber system considerably.
One approach that has been used is to place a solenoid system immediately inside the vacuum chamber, and to construct the chamber, including a cylindrical wall and end plates, with a material having good magnetic properties, such as mild steel. The solenoids in this configuration are disposed close to the inner cylindrical surface of the vacuum chamber, and if they extend end to end, a relatively uniform field can be generated. Moreover, if the cathode has weak residual magnetic characteristics an additional solenoid can be disposed inside the cathode itself. A significant disadvantage of this approach, however, is that the solenoid system contributes to off-gassed materials in the vacuum chamber, resulting in an increased pumping time for evacuation of the chamber, and possible introduction of impurities in the deposited films. Furthermore, the solenoids have to be operated at a low voltage to avoid initiating a plasma discharge around the solenoids themselves. Consequently, high solenoid currents must be used, and the solenoids must be water cooled. The introduction of water into the vacuum chamber raises the possibility of water leaks inside the chamber.
The foregoing difficulties have led to the suggestion that a double-wall structure be employed. An inner cylindrical wall forms the vacuum chamber and is joined and sealed with flat end plates, while an outer cylindrical wall forms a magnetic envelope and is also in close contact with the end plates, although not in sealing contact. A solenoid array is mounted between the inner and outer walls, and natural convection can provide sufficient cooling, since high solenoid currents are not necessary. Although this scheme is generally satisfactory for relatively small vacuum chambers oriented on a vertical axis, the design becomes mechanically awkward and too costly if the chamber is very large, or is disposed on a horizontal axis.
It will be appreciated from the foregoing that there has, prior to this invention, been a significant need for a magnetic field generation system for use in very large vacuum chambers of sputter deposition devices. The present invention satisfies this need.